1. Field of the Invention:
The present invention relates to an electronic component, particularly to a piezoelectric device having a terminal electrode between a functional substrate and a holding substrate, and a method for producing the same.
2. Description of the Related Art:
Various kinds of piezoelectric devices utilizing piezoelectric effects, such as quartz oscillators and SAW filters, have widely been used as various high-frequency filters and oscillators in radio communication equipment. These piezoelectric devices use mechanical vibration of a piezoelectric element which is generated when an electric field is applied. Thus, the performance of such piezoelectric devices greatly depends upon a method for securing the piezoelectric element without affecting the mechanical vibration. In particular, the development of methods for securing a piezoelectric element without adversely affecting piezoelectric characteristics, hermetically sealing the piezoelectric element in the piezoelectric device, and providing electrodes leading out of the piezoelectric element have been important subject.
The following methods have been known for conventional piezoelectric devices.
One method is to mechanically secure a piezoelectric element with springs, screws, etc. This method is easy to perform but it is difficult to obtain a piezoelectric device stable against thermal change, mechanical shock, etc. for a long period of time. Another method is to secure a piezoelectric element with various adhesives and conductive pastes made of an epoxy resin, a silicone resin, etc. This method also has problems. That is, since these adhesives have low heat resistance, when solder reflow treatment at a temperature around 230.degree. C. is performed in the course of the production of a piezoelectric device, the oscillation frequency of the piezoelectric element is changed. In addition, the piezoelectric device thus produced has poor long-term thermal stability under use.
Moreover, it has been known to bond the piezoelectric element to a substrate, using glass having a low melting point. Even when glass having a low melting point is used, heat treatment at a temperature of about 500.degree. C. or higher is required for obtaining a piezoelectric device with high reliability; therefore, the coefficient of thermal expansion of a substrate to which the piezoelectric element is to be bonded is required to match with that of a piezoelectric substrate of the piezoelectric device. Furthermore, materials for the piezoelectric device should be thermally stable at such a high temperature (i.e., about 500.degree. C. or higher). As one method for solving these problems, Japanese Laid-Open Patent Publication No. 4-283957 discloses a method for bonding quartz crystal (piezoelectric single crystal) to silicon through direct bonding. The piezoelectric device in which the piezoelectric element is bonded to the substrate through direct bonding is remarkably stable against thermal change and mechanical vibration. According to the method disclosed in Japanese Laid-Open Patent Publication No. 4-283957, the respective surfaces of quartz crystal and silicon to be bonded to each other are washed, the respective bonding surfaces of quartz crystal and silicon are subjected to hydrophilic treatment and bonded to each other, followed by heat treatment. The piezoelectric device produced by this method is mechanically and thermally stable; however, it is impossible to extend the terminal electrodes from the bonded portion between the piezoelectric element and the substrate, so that there are various restrictions such as the necessity of forming via holes for connecting the piezoelectric element to the other elements (see FIG. 1 of Japanese Laid-Open Patent Publication No. 4-283957).